1. Field of Invention
This invention relates to sensors utilizing optical interferometer, and particularly to high-resolution sensors utilizing multiple optical interferometers.
2. Description of Prior Art
Previously disclosed interferometric accelerometers can be made compact thanks to the technology of micro-electro-mechanical systems (MEMS) or micro-opto-electro-mechanical systems (MOEMS). But because interferometers usually involve discrete bulky components, such as focus lens, further dimension reduction is hindered. Additionally, it is also difficult to reduce the cost further due to the discrete parts. However, there exists a need for accelerometer that has a smaller size and lower price, especially in the cost-sensitive consumer market.
Optical interferometers, as a fundamental tool, are indispensable in many high-resolution measurements. In principles, an optical interferometer is capable of highly sensitive detection. In reality, however, there are several factors which limit the interferometer performance. The factors include optical power variation, wavelength shift, and refractive index change, all of which are quite common in practice and expensive to manage. In other words, it is difficult to keep the input power, wavelength, and refractive index at a constant value. For example, the output power of semiconductor laser not only varies with ambient temperature, but also declines with time in aging process. In addition, a laser's output spectrum changes because of temperature fluctuation and aging process, too. Temperature also causes change of the refractive index, which in turn may affect phase delay of a beam. Since the interference intensity is determined by the input power and phase difference and the phase difference is dependent upon the wavelength and refractive index, calibration of the input power, wavelength, and refractive index becomes critical to high-resolution interferometric measurements. Otherwise, when the interference intensity changes, it is hard to tell whether it is caused by a variable to be measured or any of the aforementioned factors. On the other hand, the current high-resolution methods for testing the parameters, especially the wavelength, are bulky and costly. Therefore, it is quite desirable to monitor the parameters, i.e. the wavelength, temperature, refractive index, and input power, in an effective yet low-cost way.